Increasing industrial application of biological process fluids, such as in waste water treatment and biological gas production, has resulted in a need for development of reliable methods for evaluation and control of the considered biological process, e.g. the anaerobic digestion process.
Monitoring of the process first of all requires access to a suitable parameter reflecting the metabolic state of the process. Secondly, it requires a system providing reliable determination of this process parameter in order to obtain efficient control, with the shortest possible delay, between process failure and operator response.
At present, measurements of suitable parameters reflecting the metabolic state of biological processes, such as waste water treatment and biological gas production, are only used to a limited extent. This is due to several factors, one of which is that filtering adequate volumes of the process fluid from a biological process, e.g. an anaerobic digestion, offers a series of problems. Most such process fluids have a high content of organic fibers, inhomogenous particles, and dissolved fats, which gives them wearing and clogging properties making them unsuitable for traditional membrane filtration. Furthermore, the high content of inhomogenous particles in these process fluids, makes it difficult to remove all particles with a diameter larger than acceptable for most analytical equipment through continuous one-step filtration, without employing very large quantities of process fluid. At present this renders the currently available methods uneconomical.
As described above, the on-line recovery of a limited amount of filtrate that can be employed in the analysis is one of the main problems when trying to develop methods for evaluation and control of biological processes, such as anaerobic digestion. The mere separation of solids from fluids is a problem faced in a diverse range of processing situations. Interestingly, the same principles that govern mineral separation in huge froth flotation plants handling millions of tons of ore also apply to separations carried out in laboratories on centiliters of a raw solution using the most sophisticated equipment. Further, the same principles apply to separations of true solutions with no particles involved (e.g., solutions with dissolved molecules such as salts, proteins, etc.). Of course, the filter media and the process conditions are different, but the basic principles are the same.
Some of the most widely described and used filtration processes are those involving porous membranes. The fluid containing solid particles passes through the pores in the membrane, and the particles are trapped on the retentate side of the filter surface. A wide variety of membrane types are available on the market. They vary by material as well as by pore size. Membranes may be woven or non-woven metallic thread, ceramic, plastic, cloth or a hybrid of two or more materials. Whatever the membrane material, the operation of the membrane can be characterized by a number of parameters, including construction, performance, pore size and porosity.
For efficient operation, the pore size of the membrane filter surface must be less than the size of the smallest particles to be removed. If these criteria are met the fluid will flow through the membrane but the particles will be captured. After a period of time the increasing amount of trapped solids will block the pores of the membrane and prevent or restrict the flow of fluid, and in turn this will lead to the main disadvantage when employing this type of filtration, namely clogging of the pores.
Once the flow of fluid through the filter is reduced below acceptable levels it is necessary to clean the filter. When using prior art filtration techniques, the filtering medium is typically cleaned for reuse by mechanical removal of solids or by backflushing. Obviously, normal filtration operations must be suspended during the cleaning operation. When backflushing, a fluid, usually water containing detergents or a suitable solvent for the solids, is passed in a reverse direction through the filter to dislodge and remove solid particles from the pores of the filtering medium after the medium has become fully or partially plugged. The disruption is minimized if the back flushing is a simple reversal of the filtration operation but more often back flushing involves a different process. Many back flush operations require steam or compressed air to be directed back through the filter. This leads to a complex system for performing all the required operations.
As noted above, the clogging or blinding of filter media is a problem at any level of filtration, insofar as the transmembrane flow drops as the pores in the filter media become clogged. While scraping off a filter cake and backflushing the canvas will suffice in simple flotation separations, the problems multiply when one deals with finer separations and in particular if one deals with an assembly for continuously withdrawing and filtering partial volumes of a process fluid containing inhomogenous particles.
In view of the foregoing it would be helpful, when considering the development of reliable methods for evaluation and control of biological processes, such as anaerobic digestion processes, to have access to a method and an assembly for continuously withdrawing and filtering partial volumes of the process fluid, directly from the container holding it and removing all particles with a diameter larger than acceptable for the analytical equipment employed. However, as will also be understood from the above, the development of an assembly for continuously in situ filtering the process fluid from biological processes, e.g. anaerobic digestions, poses several problems, since this necessitates the filtering of small quantities of process fluid with a high content of relatively large inhomogenous particles.
One known way of overcoming the deficiencies of the traditional dead end filtration described above, is by feeding fluid across a membrane from an inlet port to an outlet port, known as cross flow filtration. Filtrate is then drawn through the membrane to a filtrate outlet port. Because the feed fluid flows across the surface of the membrane the amount of material trapped permanently in the pores of the membrane is reduced. Essentially, the principle behind the inherent selfcleaning of the filtermaterial utilized in traditional cross flow techniques, would be applicable for on line filtration of biological process fluids, assuming that an appropriate system for continuously filtering and recirculating process fluid to the process fluid could be constructed. However, ordinarily cross flow filtration depends on a linear flow rate of at least 0.5–1 m/sec in order to function properly, which necessitates the use of either very narrow tubing dimensions or high capacity pumps. This is a serious limitation to this procedure, when considering biological process fluids, since these tend to have a high content of inhomogenous particles. Thus, the tubing size actually needed to avoid filter clogging severely limits the kind of fluids that can be filtered by traditional cross flow methods without the use of very large tubing dimensions. In particular process fluids with a high content of organic fibers, crystalline particles, and dissolved fats, such as the process fluid from an anaerobic digestion, have wearing and clogging properties, which would necessitate tubing resulting in the handling of large quantities of process fluid. At present this makes the currently available cross flow methods uneconomical for continuously filtering a biological process fluid containing inhomogenous particles.
Another known way of overcoming filter clogging is by employing one or several rotating parts in the filtration apparatus. U.S. Pat. Nos. 4,876,013 and 4,790,942 describe methods and various assemblies for filtration, employing an inner body rotating within a stationary outer body, on which either or both a membrane is mounted. Here attempts are made to overcome the clogging problem by making use of a hydrodynamic phenomena known as Taylor vortices, which is created in the parent fluid in the narrow gap between the bodies by the rotation. Parent fluid is feed to the gap between the cylinder surfaces, and the fluid is filtered through the membrane(s) during rotation of the inner or both cylinders, and means are provided to lead permeate from the membrane to a collection point. A serious limitation to these procedures within the present context is that they rely on an outer body within or along with, which the inner body or membranes can rotate. This puts severe limitations on the character and size of the particles contained in the fluids, which can be filtered employing this method. In particular, process fluids with a high content of organic fibers, crystalline particles, and dissolved fats, e.g. the process fluid from an anaerobic digestion, have wearing and clogging properties making them unsuitable for this type of filtration, since the content of inhomogenous particles in these process fluids would inevitably hinder the access of process fluid to the processing zone in the narrow gap. Furthermore the filtering assemblies described in U.S. Pat. Nos. 4,876,013 and 4,790,942 are, when they are to be used for continuous filtration, connected to the container holding the process fluid by tubing and can per se not be considered as true build in assemblies. Furthermore, as with the traditional cross flow techniques described above, the tubing size actually needed to avoid filter clogging severely limits the kind of fluids, which can be filtered by this method without the use of very large tubing dimensions. In particular, process fluids with a high content of organic fibers, crystalline particles, and dissolved fats, such as the process fluid from an anaerobic digestion, have wearing and clogging properties, which would necessitate tubing resulting in the handling of large quantities of process fluid.
U.S. Pat. No. 3,997,447 describes another known kind of rotary filtration structures, comprising high speed rotating disc filtration devices. This patent describes a plurality of filter discs mounted on a rotatable hollow shaft having a plurality of radial openings. The shaft is mounted for high speed rotation in a vessel or chamber which remains stationary. The fluid suspension to be filtered is admitted directly into the vessel, which indicates that the process is per se not applicable in situ, since it requires a restricted feed flow to the filtration vessel. The filtration surfaces rotate through the fluid suspension, and filtrate flows through the surfaces into the core of the filter. The filtrate flows from the filter core through the radial openings in the shaft into the fluid channels in the rotating shaft, and is then conducted out through the fluid channels for collection outside the vessel. However, when employing rotating filter discs, the fluid suspension must be in a processing zone, which is located between the shaft and the tips of the spinning discs. To gain access to this processing zone, the fluid suspension must flow radially inward past the tips and against the flow direction of solids and fluids, which are thrown radially outward from the shaft by centrifugal forces. A potential solution to this drawback is pointed out in U.S. Pat. No. 4,897,192, which describes a method similar to U.S. Pat. No. 3,997,447 apart from the fact that the process fluid to be filtered is feed to the space between two rotating discs under pressure. However, this procedure would not be applicable when handling most biological process fluids with a high content of inhomogenous particles, since as above the need for tubing seriously limits the kind of fluids, which can be filtered by this method. Hence, e.g. the process fluid from an anaerobic digestion would be unsuitable for this type of filtration.
The use of submerged tube shaped filters or filtration modules for treating process fluids is described in DE 196 25 428. The mode of operation described involves sequential filtration, which may employ rotating tube shaped filters or filtration modules for ultra- or nanofiltration. Since the method described comprises sequential submersion of the filtration module into the container holding the process fluid, the assemblies described can not be considered as permanently attached to the container and would not be suited for online filtration of partial volumes of process fluids with a high content of organic fibers, crystalline particles, and dissolved fats, e.g. the process fluid from an anaerobic digestion or of process fluids under pressure.
In conclusion all of the above mentioned prior art methods would depend either on a batch, semi-batch or at least partially off line approach in order to obtain proper filtration of small quantities of a process fluid with a high content of organic fibers, crystalline particles, and dissolved fats, such as the process fluid from an anaerobic digestion. Furthermore, when employing most of the methods there would be a need for tubing connecting the filtering device of choice to the process fluid. This would result in the handling of large quantities of process fluid, when these have a high content of organic fibers, crystalline particles, and dissolved fats, such as the process fluid from an anaerobic digestion. Hence, no means for a true on line or build in one step continuous filtration of biological process fluids with a high content of inhomogenuos particles is currently available.